Ancient Galaxy Mergers Unearthed in the Milky Way

1. Ancient merger relics discovered in the Milky Way

There are always people who laugh at astronomers who “study the universe without even understanding the earth.” But I want to say: these are completely different things, because the research methods are fundamentally different. Although the history of the Earth is not yet clear, this does not prevent astronomers from conducting archeology of the Milky Way. Unlike geology, studying the history of the Milky Way relies mainly on the analysis of large numbers of stars.

For example, the Gaia telescope we are familiar with, because it takes pictures about half a year, it can not only take pictures of stars, but also record the movement of stars. At the same time, through spectral information, it can also help us understand the elemental characteristics of each star. These, like geological information on different strata, are critical to galactic archeology.

Last month (March 2024), in an article published in the Astrophysical Journal, researchers conducted relevant analysis on millions of stars. These stars have one thing in common: they are all very old, basically over 12 billion years old. This means that these stars may have existed before the formation of the Milky Way's galactic disk. Through elemental and orbital analysis, the researchers separated the stars into two parts: Shakti, located on the outer edge of the galactic disk, and Shiva, located in the inner part.

Researchers believe that these two ancient star streams may have originally come from two separate galaxies, and then the Milky Way swallowed them up, so that those stars merged into the later Milky Way.

2. Stellar planetary recipes

Generally speaking, the two stars in a binary system are like twins. Because they were born from the same molecular cloud, they should have the same chemical characteristics. But in Gaia's data, it was discovered that the chemical characteristics of some “twins” were actually inconsistent.

Last month (March 2024), in an article published in Nature magazine, researchers used multiple large ground telescopes to conduct detailed observations of 91 pairs of special “symbiotic binary stars”. Based on the results, researchers speculate that stars with abnormal elements should be caused by engulfing surrounding planets. But if it is because the star expanded into a red giant in the later stages of evolution, it would be okay to say, but the problem is that these stars are in the main sequence stage and have not expanded very much. So how did it swallow up the surrounding planets?

Referring to the situation in the solar system, when there are terrestrial planets around the star, the rocky planet will consume a large amount of high-melting point materials such as iron, magnesium, and silicon in the protoplanetary disk during its formation. So stars with Earth-like planets are often “undernourished” compared to stars without Earth-like planets. That is, when the star is small, Earth-like planets may compete with it for food. But the problem is: in the early stages of star formation, the interior of a star is still in a state of high convection. Even if the chemical characteristics were slightly different at that time, it is difficult to detect them today.

Researchers later found through multi-body motion simulations that collisions between stars and planets were particularly frequent in the first 100 million years of their formation. On a time scale of 100 million years, stars have basically stabilized. If a planet is buried on its surface during this period, it is still possible to leave detectable traces. If the collision occurred at a later stage, it could only be caused by some external disturbance.

3. Largest supercluster

We know that superclusters are large-scale structures that are larger than galaxy groups and clusters. Since the internal parts of something like the Great Wall of Galaxy lack gravitational restraints, strictly speaking, they cannot be regarded as structures in the physical sense. Therefore, superclusters can be said to be the largest structures in the universe at present. For example, the local supercluster of galaxies in which our Milky Way is located (also known as the Virgo supercluster) has a diameter of more than 100 million light-years, and there are hundreds of galaxy clusters with ordinary internal light. If the Local Supercluster is a football field, the Milky Way is just a tiny football. However, astronomers later discovered that the local supercluster seemed to be only part of a larger galaxy cluster, the Laniakea supercluster, a behemoth with a diameter of 250 million light-years and containing more than 100,000 galaxies.

For these superclusters, previous studies have focused more on structural aspects, and little is known about how they formed. In order to solve this problem, in an article published in the Astrophysical Journal last November, researchers conducted a special study on the evolution mechanism of 662 supergalaxy clusters. The most massive of these superclusters has more than 26 trillion stars and a diameter of 360 million light-years. It can be said to be the largest supercluster ever discovered.

银河系中发现远古星系合并遗物

Through this study, it was discovered that galaxies in superclusters do indeed recede slower than the expansion of space. That is to say, under normal circumstances, galaxies will move away from each other at a relatively constant speed due to the expansion of the universe, but galaxies in superclusters do not move away from each other that fast. This is mainly because the gravitational effects between various parts of the supercluster are relatively obvious, and these parts account for 90% of the entire supercluster.

银河系中发现远古星系合并遗物

But researchers also found that the larger the supercluster, the smaller its density, and the two are almost inversely proportional to the square. In other words, the compact structure of the supercluster is only temporary, and it will gradually become looser and looser over time until it disintegrates.

4. The suddenly violent black hole

In December 2020, astronomers discovered that a black hole at the center of a galaxy located 800 million light-years away suddenly became thousands of times brighter. Why does a black hole sitting quietly at the center of a galaxy suddenly become violent? What's even more strange is that since the explosion, the black hole has not been so bright all the time, but has dimmed and brightened with a cycle of about 8.5 days, which is very regular. This process has continued for four months. So, what exactly happened there?

银河系中发现远古星系合并遗物

Last month (March 2024), in an article published in Science Advances, researchers provided an explanation for this strange phenomenon. They believe that the sudden brightening of the black hole should be caused by a star being torn apart by huge tidal forces because it was too close to the black hole (the so-called “tidal disruption event”). When a large amount of stellar material suddenly rushes into a black hole, the black hole's accretion disk lights up instantly.

So what's going on with that flickering light and dark change? Researchers speculate that there may be a “little guy” surrounding this supermassive black hole with a mass of 50 million times the sun – a medium-mass black hole with a mass of 100 to 10,000 times the sun.

银河系中发现远古星系合并遗物

This “little guy” is very close to the big black hole, about 100 times the gravitational radius of the big black hole. The key is that its orbit is almost perpendicular to the accretion disk of the large black hole, that is, it has been shuttling back and forth up and down the accretion disk. Each time a small black hole passes through an accretion disk, it ejects a violent plume of gas. Whenever the plume points in the direction of the Earth, our view is blocked, so we see the accretion disk flickering in and out periodically.

银河系中发现远古星系合并遗物

5. Origin of blue supergiant

As one of the most massive star types in the universe, blue supergiants can be called the “giants” of the stellar world, usually with dozens or even hundreds of times the mass of the sun. Such stars are so extreme that astronomers believe the conditions for their formation must have been very harsh. But in fact, whether O-type or B-type, blue supergiants are not uncommon in the universe. why is that?

银河系中发现远古星系合并遗物

Later, astronomers discovered a clue: most of these blue supergiants have no companion stars and are “singles”, which is very strange. Because according to observations, for stars about the same size as the sun, the proportion of “married” and “unmarried” people is about 50-50, and for massive stars, the proportion of “married people” can reach 75%. In other words, the larger the star, the more likely it is to have a companion star. But for blue supergiants (especially smaller B-type blue supergiants), they often do not have companion stars, which does not comply with this rule. So people speculated: Could these giants be the product of the merger of two stars?

银河系中发现远古星系合并遗物

Last month (March 2024), in an article published in the Astrophysical Journal Letters, researchers first proved this idea by analyzing 59 B-type blue supergiants in the Large Magellanic Cloud. They first re-established a new binary star merger model based on the binary star merger explanation of the supernova explosion SN 1987A. They then applied the model to these star samples and compared them, ultimately concluding that most of the existence of these blue supergiants should be caused by stellar mergers.